Delay circuit with adjustable delay

ABSTRACT

A delay circuit with adjustable delay employs a switching flip-flop which includes a differential amplifier (A) having two outputs each one feedback-looped respectively onto two inputs, so as to produce the said switching. The looping is produced by a first (AD1) and a second (AD2) adder, each having a first, a second and a third input. The first inputs are connected to corresponding outputs of the differential amplifier to produce the feedback looping. The second inputs are intended to be connected respectively to a first and a second terminal delivering a signal to be delayed (V 1   + , V 1   - ) and the third inputs receive respectively a first and a second control voltage (V 3   + , V 3   - ). The delay is a function of the difference between the first and the second control voltage. The outputs of the adders are connected respectively to corresponding inputs of the differential amplifier.

BACKGROUND OF THE INVENTION

The subject of the present invention is a delay circuit with adjustable delay employing a switching flip-flop.

According to the prior art, such a circuit employs a D-type flip-flop associated with a clock. The delay is determined by a resistance-capacitance product RC, which makes it difficult to obtain a variable delay in an integrated circuit.

The patents U.S. Pat. Nos. 4,795,923 and 4,862,020 describe integratable delay circuits whose delay is continuously adjustable with the aid of a variable voltage. These circuits require several differential stages, the outputs of which are coupled, hence resulting in relatively complicated circuits. The subject of the present invention is a circuit of the type mentioned in the first paragraph and whose delay is continuously adjustable as well as being simple to employ.

SUMMARY OF THE INVENTION

The circuit according to the invention is thus characterized in that it comprises a differential amplifier (A) having a first and a second input and a first and a second output. The first and second output are feedback-looped respectively onto the first and the second input in such a way as to produce the said switching flip-flop, the looping includes a first (AD1) and a second (AD2) adder, each adder having a first, a second and a third input as well as an output. The first inputs of the first and second adder are connected to the first and second output, respectively, of the differential amplifier to produce the said feedback looping. The second inputs are connected respectively to a first and a second terminal to receive a signal to be delayed (V₁ ⁺, V₁ ⁻) and the third inputs receive a respective first and a second control voltage (V₃ ⁺, V₃ ⁻). The delay is a function of the difference between the first and the second control voltage (V₃ ⁺, V₃ ⁻). The outputs of the first and second adders (AD1, AD2) are connected respectively to the first and second inputs of the differential amplifier (A).

According to a preferred embodiment, at least one of the said adders comprises a first and a second transistor whose base electrodes are connected respectively to the corresponding signal terminal and to the corresponding output of the differential amplifier. The main current path of each transistor is arranged in series with, in succession, respectively, a first resistor and a first current source for the first transistor and a second resistor and a second current source for the second transistor. The adder further comprises a serial branch comprising a third and a fourth resistor which are connected, on the one hand, to the point common to the first resistor and to the first current source and, on the other hand, to the point common to the second resistor and to the second current source. The first and second resistors may have the same value. The first and the second current source may have the same value of current.

The first and the second control voltage advantageously have the value V₀ +ΔV and V₀ -ΔV, respectively, ΔV being a variable whose absolute value is less than that of the constant V₀.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood from the following description which is given by way of non-limiting example with reference to the accompanying drawings, in which:

FIG. 1 shows a delay circuit according to the invention;

FIG. 2 shows an illustration of the delay function of the circuit according to the invention, and

FIGS. 3a, 3b and 4 show embodiments of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to FIG. 1, a differential amplifier A has two inputs and two outputs delivering signals V₂ ⁻ and V₂ ⁺ respectively. These two outputs are feedback-looped onto corresponding inputs through two multi-input adders AD1 and AD2 respectively. This feedback looping thus produces a switching flip-flop.

Besides the feedback input receiving the signal V₂ ⁺ , the adder AD1 has a signal input receiving a signal V₁ ⁺ and a control input receiving a d.c. voltage level V₃ ⁺ which can be varied.

Besides the feedback input receiving the signal V₂ ⁻, the adder AD2 has a signal input receiving a signal V₁ ⁻ and a control input receiving a d.c. voltage level V₃ ⁻ which can be varied.

The signals V₁ ⁻ and V₁ ⁻ are representative of the two polarities of the input signal to be delayed. The signals V₃ ⁺ and V₃ ⁻ are employed to obtain the offset operation of the switching flip-flop and their value influences the value of the delay Δt. FIG. 2 illustrates the delay value Δt for squarewave signals.

It is possible, for example, to set:

    V.sub.3.sup.+ =V.sub.0 +ΔV

    V.sub.3.sup.- =V.sub.0 -ΔV

V₀ =constant voltage.

ΔV=variable voltage (positive, negative or zero) of absolute value less than that of V₀.

The control signals may be direct control voltages inserted at the third inputs of voltage adders.

FIGS. 3a and 3b represent a preferred embodiment of the adders AD1 and AD2, for which the voltages V₃ ⁺ and V₃ ⁻ come in by way of current sources. The control signals are then, for example, voltages controlling the intensity of the current sources.

According to FIG. 3a, the adder AD1 comprises two npn transistors T₁ and T₂ whose collectors are connected to a voltage supply source V_(cc), whose bases receive the signals V₁ ⁺ and V₂ ⁺ respectively, and each of whose emitter is connected to a branch comprising, in series, for the transistor T₁, a resistor R₁ and a variable current source I₁, and for the transistor T₂, a resistor R₄ in series with a variable current source I₂.

Between the point A common to the resistor R₁ and to the variable current source I₁, and the point B common to the resistor R₄ and to the variable current source I₂, are arranged two resistors in series R₂ and R₃ whose common point C, which delivers the signal V_(in) ⁺ is connected to one input of the amplifier A.

The current from the current sources I₁ and I₂ may be controlled by a voltage, preferably the same voltage. This enables production in integrated circuit form.

According to FIG. 3b, the adder AD2 comprises two npn transistors T₃ and T₄ whose collectors are connected to the voltage supply source V_(cc), whose base receives the signals V₂ ⁻ and V₁ ⁻ , respectively and each of whose emitter is connected to a branch comprising, in series, for the transistor T₃, a resistor R₅ and a variable current source I₃, and for the transistor T₄, a resistor R₈ and a variable current source I₄.

Between the point D common to the resistor R₅ and to the variable current source I₃, and the point E common to the resistor R₈ and to the variable current source I₄, are arranged two resistors in series R₆ and R₇ whose common point F, which delivers the signal V_(in) ⁻ is connected to the other input of the amplifier A. The current from the current sources I₃ and I₄ may be controlled by one and the same voltage.

FIG. 4 shows a preferred embodiment in which the adders AD1 and AD2 both conform to the deployments of FIGS. 3a and 3b (with the same reference labels) and in which the amplifier A consists of a differential stage including two transistors T₁₀ and T₁₁ whose coupled emitters are connected to a current source I. The collectors of the transistors T₁₀ and T₁₁ are connected to the voltage supply source V_(cc) through resistors R₁₀ and R₁₁ respectively, and their bases are connected to the points C and F, respectively. The collector of the transistor T₁₀, which delivers the output signal V₂ ⁻ , is connected to the base of the transistor T₃, and the collector of the transistor T₁₁ which delivers the output signal V₂ ⁺ , is connected to the base of the transistor T₂.

The basic idea of the invention consists in varying the quiescent point of the inputs of the differential amplifier in order to vary the delay Δt. In other words, the points C and F are brought to d.c. potentials which can be varied, and the delay depends, as will now be shown, on the difference between these potentials.

Let I_(a) (FIG. 3a) be the current flowing in the resistor R₁.

Let V_(BE1) be the base-emitter voltage of the transistor T₁ and V_(BE2) that of the transistor T₂. Neglecting the base resistances and the base currents, we have:

    V.sub.1.sup.+ -V.sub.BE1 -R.sub.1 I.sub.a -(I.sub.a -I.sub.1) (R.sub.2 +R.sub.3)=V.sub.2.sup.+ -V.sub.BE2 -R.sub.4 (I.sub.1 +I.sub.2 -I.sub.1)

and

    V.sub.BE1 -V.sub.BE2 =V.sub.T LOG {I.sub.a /(I.sub.1 +I.sub.2 -I.sub.a)}V.sub.T =26 mV

whence

    V.sub.BE1 ≃V.sub.BE2 if I.sub.a ≃(I.sub.1 +I.sub.2)/2

this condition will be explained further on. By assumption, we choose:

    R.sub.1 =R.sub.4 R.sub.2 =R.sub.3 I.sub.1 =I.sub.2 =I.sub.0 +ΔI

We then have: ##EQU1## which corresponds to a value of V₃ ⁺

    V.sub.3.sup.+ =2R.sub.1 (I.sub.0 +ΔI)

(the adder AD1 has a gain of 0.5)

    V.sub.BE1 ≃V.sub.BE2 if I.sub.a ≃I.sub.1 that is (V.sub.1.sup.+ -V.sub.2.sup.+)<<2(R.sub.1 +R.sub.2)I.sub.1

In the same way, for the diagram of FIG. 3b and with the assumptions R₅ =R₈ and R₆ =R₇, I₃ =I₄ =I₀ -ΔI, we obtain: ##EQU2## whence

    V.sub.3.sup.- =-2R.sub.5 (I.sub.0 -ΔI). (adder of gain 0.5).

    V.sub.BE3 V.sub.BE4 if (V.sub.1.sup.- -V.sub.2.sup.-) <<2 (R.sub.5 +R.sub.6) I.sub.3

V_(BE3) and V_(BE4) designating the base-emitter voltages of the transistors T₃ and T₄.

The equations (1) and (2) then give (on choosing R₁ =R₅) ##EQU3## ΔV_(in) represents the offset of the operating point of the switching flip-flop. It varies linearly as a function of ΔI.

In accordance with the equations governing the differential stages, we have: ##EQU4## with R₁₀ =R₁₁, th=hyperbolic tangent function. On equilibrium of the differential stage (V_(in) ⁺ =V_(in) ⁻), V₁ ⁺ is not equal to V₁ ⁺.

In fact, we then have:

    V.sub.1.sup.+ -V.sub.1.sup.- =4R.sub.1 ΔI+2V.sub.T LOG {(I.sub.0 +ΔI)/ I.sub.0 -ΔI)}

The difference between V₁ ⁺ and V₁ ⁻ at equilibrium of the differential stage depends only on ΔI, that is to say on the difference between the currents in the adders of FIG. 3a and 3b.

The formula giving the response time at 50%, denoted t₅₀, of the amplitude is as follows: t₅₀ =4V_(T) /R₁₀ Iτ Log (2+R₁₀ I/4V_(T) +R₁₀ I/VT .ΔI/ΔV₁) t₅₀ may be taken as the characteristic value of the delay, with V₁ -peak-to-peak amplitude of the signal to be delayed, and τ=intrinsic response time of the differential stage, ##EQU5## C=capacitance of the differential stage in its equivalent diagram. (one capacitance C for each of the collectors of T₁₀ and T₁₁).

The above calculations were performed with certain assumptions (R_(l) =R₄ =R₅ =R₈, R₂ =R₃ and R₆ =R₇), but of course this did not involve necessary conditions. In particular, the ratios between R₁ and R₄, R₅ and R₈, on the one hand, and between R₂ and R₃, and R₆ and R₇ on the other hand, influence the symmetry of the waveform, that is to say enable the obtainment of a rise time equal to or different from the fall time. Different response times can in fact be obtained on each input, if AD1 and AD2 are different.

For R₂ <R₃, a phase advance is obtained for the corresponding input and a phase delay for R₂ >R₃.

For R₁ <R₄, a phase advance is obtained for the corresponding input and a phase delay for R₁ >R₄.

Similar reasoning applies to the ratios between R₅ and R₈ on the one hand, and R₆ and R₇ on the other hand.

In fact, the values of the said resistors influence the value of the difference between V_(in) ⁺ and V_(in) ⁻ on the one hand and on the value of τ on the other hand.

Under the assumption that the two devices AD1 and AD2 are structurally identical, we have in fact: ##EQU6##

On inverting the formula cited earlier and giving V₂ ⁺ -V₂ ⁻ as a function of V_(in) ⁺ -V_(in) ⁻ we have:

    V.sub.in.sup.+ -V.sub.in.sup.- =2V.sub.T arc th {2(V.sub.2.sup.+ -V.sub.2.sup.-) / R.sub.10 I }

arc th=inverse hyperbolic tangent function. whence: ##EQU7## The hysteresis condition may be written: ##EQU8## whence the operating condition 

We claim:
 1. A delay circuit with adjustable delay employing a switching flip-flop which comprises: a differential amplifier having a first and a second input and a first and a second output, the first and second output being feedback-looped respectively onto the first and the second input so as to produce said switching flip-flop, the looping including a first and a second adder, each adder having a first, a second and a third input and an output, said first inputs of the first and second adder being connected to respectively the first and second output of the differential amplifier to produce said feedback looping, the second inputs being connected respectively to a first and a second terminal to receive a signal to be delayed (V₁ ⁺, V₁ ⁻) and the third inputs being adapted to receive a first and a second control voltage (V₃ ⁺ V₃ ⁻), said delay being a function of the difference between the first and the second control voltage (V₃ ⁺, V₃ ⁻), and the outputs of the first and second adders being connected respectively to the first and second inputs of the differential amplifer.
 2. A delay circuit asccording to claim 1, wherein at least one of the adders comprises a first and a second transistor having a base connected respectively to the corresponding terminal and to a corresponding one of said first and second terminals output of the differential amplifier and with a main current path of the first transistor connected in series with a first resistor and a first current source and with a main current path of the second transistor connected in series with a second resistor, and a serial branch comprising a third and a fourth resistor connected to a point common to the first resistor and to the first current source and to a point common to the second resistor and to the second current source.
 3. A delay circuit according to claim 2, wherein first and second resistors have substantially the same resistance value and the first and second current sources have substantially the same value of current.
 4. A delay circuit according to claim 3 wherein the third and fourth resistors have substantially the same resistance value.
 5. A delay circuit according to claim 4 wherein the first and second control voltage have a value V₀ +ΔV and V₀ -ΔV respectively, ΔV being a variable whose absolute value is less than V₀.
 6. A delay circuit according to claim 2 wherein the third and fourth resistors have substantially the same resistance value.
 7. A delay circuit according to claim 1 wherein the first and second control voltage have a value V₀ +ΔV and V₀ -ΔV respectively, ΔV being a variable whose absolute value is less than V₀.
 8. A delay circuit according to claim 2 wherein the first and second control voltage have a value V_(o) +ΔV and V₀ -ΔV respectively, ΔV being a variable whose absolute value is less than V₀. 